KeepingYouAwake is nice (and I'm not about to pretend the following solution is good for everyone), however, I've found on the last couple of McBook fresh installs using caffeinate [1] from the terminal is sufficient.
I usually have a terminal open anyway and `caffeinate -d -t 999999` usually does what I want (stay awake until I come back and tell you to go to sleep). Usually it's two keystrokes away (up arrow, enter), if not, that's quick enough to type for my purposes. That said, it's a bad approach if you want to suggest it as a tool for someone's workflow who _doesn't_ have a terminal open all of the time.